NEW ORLEANS – Underpromising with hopes of overdelivering, BP said Sunday that it is making progress on what could prove its most effective effort yet to contain the Gulf oil leak, but cautioned that the verdict could be several days away.

A new cap being placed atop the gusher is intended to provide a tight seal and might eventually allow the oil giant to capture all the crude leaking from the well for the first time since an April 20 oil rig explosion set off the environmental crisis. But several prior failed attempts to stop the leak have made BP PLC careful to keep expectations grounded.

“We’re pleased with our progress,” said BP senior vice president Kent Wells, who then hastened to add the operation was still expected to last up to six more days.

Asked during a conference call if the new cap and collection efforts would end the spilling of oil into the Gulf, Well said only that BP will capture all the oil “at some point.”

Wells said BP may have to bring another vessel back online and add additional collection capacity in order to stop the oil flow altogether. Officials won’t be satisfied the cap is working until they’ve run tests, Wells said.